Semiconductor laser

ABSTRACT

A semiconductor laser having a high electrostatic withstand voltage, resistant to a power supply surge, and having improved long-term reliability is obtained by reducing current leakage through a threading dislocation portion. The semiconductor laser includes a substrate having a high dislocation region having a dislocation density of 1×10 5  cm −2  or more, a crystalline semiconductor structure located on the substrate and having an active layer, an insulating film located on the semiconductor structure, a surface electrode located on the insulating film and electrically continuous with the semiconductor structure for injection of a current into the active layer, and a back electrode located on a rear surface of the substrate. The semiconductor laser has a laser resonator with a length L, and the area of the surface electrode is 120×L μm 2  or less.

TECHNICAL FIELD

The present invention relates to a semiconductor laser made on asubstrate of a high threading dislocation density, e.g., a GaNsubstrate.

BACKGROUND ART

GaAs substrates and InP substrates have been used as substrates used forsemiconductor light emitting elements and semiconductor electronicdevices (see, for example, patent document 1). On the other hand, inrecent years, semiconductor light emitting elements and semiconductorelectronic devices using substrates such as GaN substrates having athreading dislocation density much higher than those of GaAs and InPsubstrates have been made and put to practical use. For example, as suchdevices, semiconductor lasers and light emitting diodes using III-Vgroup nitride compound semiconductors and capable of light emission froma blue region to an ultraviolet region are known.

Patent Document 1: Japanese Patent Laid-Open No. 6-37386

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, it is known that the possibility of malfunction due to a powersupply surge or static electricity in such devices is much stronger thanthat in semiconductor light emitting devices using a GaAs substrate oran InP substrate. The reason for this has not been made clear but athreading dislocation existing in the substrate has been found to be amajor factor. It has also been found that not only a threadingdislocation in a light emitting portion, i.e., a current injectionportion, in a semiconductor laser or a light emitting diode but also athreading dislocation in a portion other than the current injectionportion, i.e., a current non-injection portion, is a major factor indegradation or malfunction. In semiconductor lasers in general, thisphenomenon is noticeable because the current injection portion is muchsmaller than the current non-injection portion.

FIG. 19 is a sectional view of a conventional semiconductor laser havinga ridge waveguide. As shown in the figure, a semiconductor layer 102 ofa III-V group nitride compound is formed on a GaN substrate 101. Also, acapacitor is formed by an insulating film 103 for blocking a current anda semiconductor layer 102 and an electrode 104 between which theinsulating film 103 is sandwiched. As a result, the semiconductor laserhas an extremely low impedance with respect to an instantaneous changein voltage due to a power supply surge or static electricity, i.e., anextremely high frequency component. Also, pin holes and a locally thinportion exist in the insulating film 103 and the impedance is furtherreduced at such portions. Further, a threading dislocation 105 has asubstantially low resistance in comparison with other portions due toits structural defect.

Therefore, if a low-impedance portion of the insulating film 101 isadjacent or close to the threading dislocation 105, a current flowsthrough a path: surface electrode 104, insulating film 103, threadingdislocation 105 and back electrode 106. Since the threading dislocation105 is extremely narrow, the current density thereof becomes extremelyhigh if current are concentrated thereon. Destruction of thesemiconductor layer 102 or breakdown of the insulating film 103 on thesemiconductor layer 102 results. A steady current leak path is therebyformed in this portion, which makes it difficult to cause a current toflow to the light emitting layer through the current injection portion107. A problem arises that a degradation in performance or malfunctionof the device occurs.

In GaN substrates presently put to practical use and sold on the market,a low dislocation region in which the threading dislocation density iscomparatively low and a high dislocation region which is formed instripe form in the low dislocation region and in which the threadingdislocation density is high exist. This high dislocation region isobserved as a region in stripe form apparently different in contrastfrom other regions and having a width of several microns to several tenmicrons, when seen from the substrate upper surface or the substratelower surface. The above-described problem occurs easily with the highdislocation region or in a region in the vicinity of the highdislocation region because threading dislocations exist densely therein.In semiconductor lasers using a GaN material in particular, theoccurrence of the above-described problem is increased since theresistance of the p-type layer is high, and since the series resistanceof the current injection portion is high.

The present invention has been achieved to solve the above-describedproblem, and an object of the present invention is to obtain asemiconductor laser in which a current leak through a threadingdislocation portion is reduced to improve the long-term reliability,surge resistance and static electricity resistance.

ADVANTAGES OF THE INVENTION

According to the present invention, a semiconductor laser can beobtained in which a current leak through a threading dislocation portionis reduced to improve the long-term reliability, surge resistance andstatic electricity resistance.

BRIEF DESCRIPTION OF DRAWING FIGURES

FIG. 1 is a plan view of a semiconductor laser used in experimentation;

FIG. 2 is a graph relating surface electrode area to withstand voltageof a semiconductor laser;

FIG. 3 is a graph relating area near a center of a high dislocationregion and withstand voltage of a semiconductor laser;

FIG. 4 is a graph relating area near a center of a high dislocationregion and withstand voltage of a semiconductor laser;

FIG. 5 is a graph relating back surface electrode area to withstandvoltage of a semiconductor laser;

FIG. 6 is a graph relating area near a center of a high dislocationregion and withstand voltage of a semiconductor laser;

FIG. 7 is a graph relating area near a center of a high dislocationregion and withstand voltage of a semiconductor laser;

FIG. 8 is a cross-sectional view of a semiconductor laser according toan embodiment of the invention;

FIG. 9 is a top view of the semiconductor laser of FIG. 8;

FIG. 10 is a top view of a semiconductor laser according to anembodiment of the invention;

FIG. 11 is a top view of a semiconductor laser according to anembodiment of the invention;

FIG. 12 is a top view of a semiconductor laser according to anembodiment of the invention;

FIG. 13 is a top view of a semiconductor laser according to anembodiment of the invention;

FIG. 14 is a bottom view of a semiconductor laser according to anembodiment of the invention;

FIG. 15 is a bottom view of a semiconductor laser according to anembodiment of the invention;

FIG. 16 is a bottom view of a semiconductor laser according to anembodiment of the invention;

FIG. 17 is a bottom view of a semiconductor laser according to anembodiment of the invention;

FIG. 18 is a bottom view of a semiconductor laser according to anembodiment of the invention;

FIG. 19 is a cross-sectional view of a conventional semiconductor laser.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

A current leak through a threading dislocation portion occurs only at aportion at which a surface electrode and a back electrode are formed.Therefore, the current leak through the threading dislocation portioncan be reduced if the area of the surface electrode or the backelectrode is reduced. An experiment on the relationship between the areaof the surface electrode or the back electrode and the electrostaticwithstand voltage was made as described below. The electrostaticwithstand voltage is the average of breakdown voltages when a current iscaused to flow in the forward direction in a machine model.

FIG. 1 is a top view of a semiconductor laser used in the experiment. Asthis semiconductor laser, three types of lasers respectively havingresonator lengths L of 400, 600 and 800 μm by cleavage were formed. Highdislocation regions 14 exist in the GaN substrate 1 at intervals of 400μm. The chips were separated so as to have a width of 400 μm along thehigh dislocation regions 14. Black round marks in the figureschematically indicate positions at which threading dislocations existin the GaN substrate and the semiconductor layer.

FIG. 2 shows the relationship between the area of the surface electrodeand the electrostatic withstand voltage when the surface electrode widthw (μm) was changed in the semiconductor laser. From the results shown inFIG. 2, it was found that the electrostatic withstand voltage wasincreased with the reduction in the area of the surface electrode. Thereason for this is thought to be that the probability of generation of aleak current from a pin hole in the insulating film or a portion of asmall film thickness or low film quality through a threading dislocationis reduced with the reduction in the electrode area. It was also foundthat the electrostatic withstand voltage was reduced with the reductionin resonator length L. The reason for this is thought to be that theresistance value of the essential current injection portion is increasedwith the reduction in resonator length L, and that the occurrence of acurrent leak is increased thereby. It was also found from a long-termlife test that the life was increased with the reduction in the area ofthe surface electrode.

Therefore, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained by reducing the area of the surfaceelectrode. More specifically, the area of the surface electrode is setto 120×L μm² or less, more preferably 80×L μm² or less, and furtherpreferably 60×L μm² or less.

FIG. 3 shows the relationship between the area of a portion within arange of 100 μm from a center of the high dislocation region on oppositesides of the same in the surface electrode and the electrostaticwithstand voltage, and FIG. 4 shows the relationship between the area ofa portion within a range of 60 μm and the electrostatic withstandvoltage. From these results, it was found that the electrostaticwithstand voltage of the portion within the 100 μm range from the centerof the high dislocation region was reduced relative to that of otherportions, and that the electrostatic withstand voltage of the portionwithin the 60 μm range was further reduced. The reason for this isthought to be that the threading dislocation density is originally highin the vicinity of the high dislocation region and, in addition,threading dislocations increased from the high dislocation region to aportion in the low dislocation region with the progress in crystalgrowth of the semiconductor layer on the GaN substrate.

Therefore, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained by placing the surface electrode as remotelyas possible from the vicinity of the high dislocation region or byminimizing the area of the surface electrode in the vicinity of the highdislocation region. More specifically, the area of the portion withinthe 100 μm range from the center of the high dislocation region on theopposite sides of the same in the surface electrode is set to 60×L μm²or less, preferably 45×L μm² or less. Also, the area of the portionwithin the 60 μm range is set to 20×L μm² or less, preferably 15×L μm²or less, and further preferably 10×L μm² or less.

FIG. 5 shows the relationship between the area of the back electrode andthe electrostatic withstand voltage. From the results shown in FIG. 5,it was found that the electrostatic withstand voltage was increased withthe reduction in the area of the back electrode as in the case ofsurface electrode.

Therefore, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained by reducing the area of the back electrode.More specifically, the area of the back electrode is set to 140×L μm² orless, more preferably 100×L μm² or less, and further preferably 60×L μm²or less.

FIG. 6 shows the relationship between the area of a portion within arange of 100 μm from a center of the high dislocation region on oppositesides of the same in the back electrode and the electrostatic withstandvoltage, and FIG. 7 shows the relationship between the area of a portionwithin a range of 60 μm and the electrostatic withstand voltage. Fromthese results, it was found that the electrostatic withstand voltage ofthe portion within the 100 μm range from the center of the highdislocation region was reduced relative to that of other portions, andthat the electrostatic withstand voltage of the portion within the 60 μmrange was further reduced.

Therefore, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained by placing the back electrode as remotely aspossible from the vicinity of the high dislocation region or byminimizing the area of the back electrode in the vicinity of the highdislocation region. More specifically, the area of the portion withinthe 100 μm range from the center of the high dislocation region on theopposite sides of the same is set to 80×L μm² or less, preferably 65×Lμm² or less. Also, the area of the portion within the 60 μm range is setto 20×L μm² or less, preferably 45×L μm² or less, and further preferably25×L μm² or less.

The present invention can be applied to semiconductor lasers using othersubstrates in which a high dislocation region having a threadingdislocation density of 1×10⁵ cm⁻² or higher as well as to semiconductorlasers using the GaN substrate exists, and the same effects can beachieved in application to such semiconductor lasers.

A semiconductor laser according to First Embodiment of the presentinvention will be described with reference to the drawings. FIG. 8 is asectional view of a nitride semiconductor laser according to FirstEmbodiment of the present invention seen from a laser emission surface.However, only a light emitting portion in the vicinity of a ridge isshown. In this semiconductor laser, an n-type AlGaN clad layer 2, ann-type GaN light guide layer 3, an active layer 4, which is a lightemitting layer, a p-type AlGaN electron barrier layer 5, a p-type GaNlight guide layer 6, a p-type AlGaN clad layer 7 and a p-type GaNcontact layer 8 are successively formed as a semiconductor crystal on aGa surface which is a major surface of a GaN substrate 1.

The GaN substrate 1 has a thickness of 100 μm. A high dislocation regionhaving a dislocation density of 1×10⁵ cm⁻² or higher is formed in stripeform in the GaN substrate 1. The n-type AlGaN clad layer 2 is formed ofan AlGaN material having an Al composition ratio of 0.07, has athickness of 1 μm, and is doped with, for example, Si provided as ann-type impurity. The n-type GaN light guide layer 3 is formed of anAlGaN material and as a thickness of 100 nm.

The active layer 4 has a double quantum well structure in which anundoped n-side InGaN-SCH layer having an In composition ratio of 0.02,an undoped InGaN well having an In composition ratio of 0.12, an undopedInGaN barrier layer having an In composition ratio of 0.02, an undopedInGaN well layer having an In composition ratio of 0.12 and an undopedp-side InGaN-SCH layer having an In composition ratio of 0.02 arestacked in order. The thickness of the n-side InGaN-SCH layer is 30 nm,the thickness of the InGaN well is 50 nm, and the thickness of the InGaNbarrier layer is 8.0 nm.

The p-type AlGaN electron barrier layer 5 is formed of an AlGaN materialhaving an Al composition ratio of 0.2, has a thickness of 20 nm, and isdoped with Mg provided as a p-type impurity. The p-type GaN light guidelayer 6 is formed of an AlGaN material and has a thickness of 100 nm.The p-type AlGaN clad layer 7 is formed of an AlGaN material having anAl composition ratio of 0.07 and has a thickness of 400 nm. The p-typeGaN contact layer 8 is formed of a GaN material and has a thickness of100 nm.

A ridge 9 for guiding a light wave is formed in the <1-100> orientationas a portion in the p-type AlGaN clad layer 7 and the p-type GaN contactlayer 8 by etching. The width of the ridge 9 is 1.5 μm. The ridge 9 isformed in a low dislocation region between high dislocation regionsformed in stripe form in The GaN substrate and having a width of severalmicrons to several ten microns.

An insulating film 10 formed of SiO₂ and having a thickness of 200 nm isformed for surface protection and electrical insulation on side surfacesof the ridge 9 and on the p-type AlGaN clad layer 7 at the side of theridge 9. An opening 11 is formed in the insulating film 10 on an uppersurface portion of the ridge 9. A p-type surface electrode 12 and thep-type GaN contact layer 8 have electrical conduction therebetweenthrough the opening 11. The surface electrode 12 is an electrode forinjecting a current only into a partial region in the active layer 4 bycurrent blocking with the insulating film 10, and has a structure inwhich Pd film and Au film is laid one on another.

An n-type back electrode 13 is formed on an N surface opposite from theone major Ga surface of the GaN substrate 1. The back electrode 13 has astructure in which Ti film and Au film are laid one on another.

FIG. 9 is a top view of the semiconductor laser according to FirstEmbodiment of the present invention. Laser element are formed bycleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. High dislocation regions 14 exist in the GaN substrateat intervals of 400 μm. The elements are separated along the highdislocation regions 14. High dislocation regions 14 therefore exist onopposite sides of each element.

The surface electrode 12 has a length of 600 μm in the resonatordirection but has an extremely small width of 80 μm. If the resonatorlength is L (μm), the area of the electrode is 80×L μm². On the otherhand, the back electrode 13 is formed substantially through the entirearea of the lower surface of the semiconductor laser.

A method of manufacturing the semiconductor laser according to thisembodiment will now be described. N-type AlGaN clad layer 2, n-type GaNlight guide layer 3, active layer 4 including an undoped n-sideInGaN-SCH layer, an undoped InGaN/InGaN double quantum well active layerand an undoped p-side InGaN-SCH layer, p-type AlGaN electron barrierlayer 5, p-type GaN light guide layer 6, p-type AlGaN clad layer 7 andp-type GaN contact layer 8 are successively formed on GaN substrate 1having its surface cleaned in advance by thermal cleaning or the likeusing a metal organic chemical vapor deposition (MOCVD).

The temperatures at which these layers are grown are, for example, atemperature of 1000° C. at which n-type AlGaN clad layer 2 and n-typeGaN light guide layer 3 are grown, a temperature of 780° C. at which thelayers from the undoped n-side InGaN-SCH layer to the undoped p-sideInGaN-SCH layer are grown, and a temperature of 1000° C. at which thelayers from p-type AlGaN electron barrier layer 5 to p-type GaN contactlayer 8 are grown.

A resist is applied to the entire substrate surface after the completionof the above-described crystal growth, and a resist patterncorresponding to the shape of ridge 9 is formed by lithography. Etchingto an internal portion of p-type AlGaN clad layer 7 is performed, forexample, by a RIE method, with this resist pattern used as a mask. Ridge9 is formed as an optical waveguide structure by this etching. In thisetching, chlorine based gas for example is used as etching gas.

An insulating film 10 formed of SiO₂ and having a thickness of 0.2 μm isformed on the entire substrate surface by, for example, a CVD method, avacuum deposition method or a sputtering method without removing theresist pattern used as a mask. Thereafter, a lift-off step in which theSiO₂ film on ridge 9 is removed simultaneously removal of the resist isperformed. Opening 11 is thereby on ridge 9.

Pt film and Au film are successively formed on the entire substratesurface, for example, by a vacuum deposition method. Thereafter, surfaceelectrode 12 is formed by resist application, lithography and wetetching or dry etching.

Thereafter, Ti film and Al film are successively formed on the entireback surface of the substrate by a vacuum deposition method, therebyforming back electrode 13. Back electrode 13 is formed into a desiredpattern by resist application, lithography and wet etching or dryetching. Alloying for making an ohmic contact is thereafter performed.

Thereafter, the substrate is worked into a bar form by cleavage or thelike to form two resonator end surfaces. Further, an end surface coatingis formed on the resonator end surfaces, and the bar is formed into achip by cleavage or the like. The semiconductor laser according to thisembodiment is thus manufactured.

The electrostatic withstand voltages of the semiconductor laseraccording to this embodiment and a conventional semiconductor laserhaving a surface electrode area of 380×L μm² were measured. The averageelectrostatic withstand voltage of the semiconductor laser according tothis embodiment was 350 V, while that of the conventional semiconductorlaser was 174 V. Thus, an improvement in electrostatic withstand voltagein the semiconductor laser according to this embodiment was observed.The reason for this is thought to be that if the electrode area isreduced, the probability of generation of a leak current from a pin holein the insulating film or a portion of a small film thickness or lowfilm quality through a threading dislocation and the back electrode isreduced.

While the description has been made of the case of using a GaNsubstrate, the semiconductor laser according to this embodiment also hasthe same advantage in a case where a substrate having a high dislocationregion of a dislocation density higher than 1×10⁵ cm⁻², and it isparticularly advantageous if the dislocation density in the substrate isincreased.

While in this embodiment the surface electrode 12 is a p-type electrode,it is an n-type electrode in a case where an insulating film and asurface electrode are formed on an n-type semiconductor by using ap-type substrate.

Second Embodiment

FIG. 10 is a top view of a semiconductor laser according to SecondEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. Separation is performed at a position shifted by 50 μmfrom a center of a high dislocation region 14. Therefore, only one highdislocation region 14 exists in the semiconductor laser. A surfaceelectrode 12 is formed at a position distanced by 80 μm from the centerof the high dislocation region 14. In other respects, the structure ofthis semiconductor laser is the same as that in First Embodiment.Therefore no further description of the structure will be made. As aresult, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained.

Third Embodiment

FIG. 11 is a top view of a semiconductor laser according to ThirdEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 200 μm. The element is separated at a center of a highdislocation region 14 and at a center between high dislocation regions14. Therefore, the high dislocation region 14 exists only at one side ofthe element. A surface electrode 12 is formed at a position distanced by80 μm from the center of the high dislocation region 14. In otherrespects, the structure of this semiconductor laser is the same as thatin First Embodiment. Therefore no further description of the structurewill be made. As a result, a semiconductor laser having a highelectrostatic withstand voltage, resistant to a power supply surge andhaving improved long-term reliability can be obtained.

Fourth Embodiment

FIG. 12 is a top view of a semiconductor laser according to FourthEmbodiment of the present invention. This semiconductor laser has, inaddition to the construction in Third Embodiment, an electrode padportion 15 for wiring on the surface electrode 12. The electrode padportion 15 has a width of 50 μm and length of 50 μm. The area of thesurface electrode 12 existing in the portion within a range of 60 μmfrom the center of the high dislocation region on the opposite sides ofthe same in the surface electrode is 1500 μm² (2.5×L μm² if theresonator length is L (μm)). As a result, a semiconductor laser having ahigh electrostatic withstand voltage, resistant to a power supply surgeand having improved long-term reliability can be obtained.

Fifth Embodiment

FIG. 13 is a top view of a semiconductor laser according to FifthEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. The element is separated at positions on highdislocation regions 14. Therefore, high dislocation regions 14 exist atopposite sides of the element.

A surface electrode 12 for supplying a current has a length of 600 μm inthe resonator direction but has an extremely small width of 10 μm. Thearea of the surface electrode 12 is 100×L μm² if the resonator length isL (μm).

Further, surface dummy electrodes 16 electrically insulated from thesurface electrode 12 are formed at certain distances from the surfaceelectrode 12 on the insulating film. No current is caused to flowthrough the surface dummy electrodes 16. At the time of bonding of thesurface electrode 12 side to a sub mount by soldering, not only thesurface electrode 12 but also the surface dummy electrodes 16 are bondedto the sub mount. Therefore, the bonding area is increased and thestability with which the element is bonded can be improved.

Sixth Embodiment

FIG. 14 is a bottom view of a semiconductor laser according to SixthEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. High dislocation regions 14 exist at intervals of 400μm on a GaN substrate. The element is separated along the highdislocation regions 14. A back electrode 13 has a length of 600 μm inthe resonator direction but has an extremely small width of 80 μm. Thearea of the back electrode 13 is 80×L μm² if the resonator length is L(μm).

The electrostatic withstand voltages of the semiconductor laseraccording to this embodiment and a conventional semiconductor laserhaving a back electrode area of 380×L μm² were measured. The averageelectrostatic withstand voltage of the semiconductor laser according tothis embodiment was 365 V, while that of the conventional semiconductorlaser was 196 V. Thus, an improvement in electrostatic withstand voltagein the semiconductor laser according to this embodiment was observed.The reason for this is thought to be that if the electrode area isreduced, the probability of generation of a leak current from a surfaceelectrode, a pin hole in the insulating film or a portion of a smallfilm thickness or low film quality through a threading dislocation andthe back electrode is reduced.

While the description has been made of the case of using a GaNsubstrate, the semiconductor laser according to this embodiment also hasthe same advantage in a case where a substrate having a high dislocationregion of a dislocation density higher than 1×10⁵ cm⁻², and it isparticularly advantageous if the dislocation density in the substrate isincreased.

The back electrode 13 is an n-type electrode in a case where an n-typesubstrate is used. The back electrode 13 is a p-type electrode in a casewhere a p-type substrate is used.

Seventh Embodiment

FIG. 15 is a bottom view of a semiconductor laser according to SeventhEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. Separation is performed at a position shifted by 50 μmfrom a center of a high dislocation region 14. Therefore, only one highdislocation region 14 exists in the semiconductor laser. A backelectrode 13 is formed at a position distanced by 80 μm from the centerof the high dislocation region 14. In other respects, the structure ofthis semiconductor laser is the same as that in Sixth Embodiment.Therefore no further description of the structure will be made. As aresult, a semiconductor laser having a high electrostatic withstandvoltage, resistant to a power supply surge and having improved long-termreliability can be obtained.

Eighth Embodiment

FIG. 16 is a bottom view of a semiconductor laser according to EighthEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 200 μm. The element is separated at a center of a highdislocation region 14 and at a center between high dislocation regions14. Therefore, the high dislocation region 14 exists only at one side ofthe element. Aback electrode 13 is formed at a position distanced by 80μm from the center of the high dislocation region 14. In other respects,the structure of this semiconductor laser is the same as that in SixthEmbodiment. Therefore no further description of the structure will bemade. As a result, a semiconductor laser having a high electrostaticwithstand voltage, resistant to a power supply surge and having improvedlong-term reliability can be obtained.

Ninth Embodiment

FIG. 17 is a bottom view of a semiconductor laser according to NinthEmbodiment of the present invention. This semiconductor laser has, inaddition to the construction in Eighth Embodiment, an electrode padportion 17 for wiring on the back electrode 13. The electrode padportion 17 has a width of 50 μm and length of 50 μm. The area of theback electrode 13 existing in the portion within a range of 60 μm fromthe center of the high dislocation region on the opposite sides of thesame in the back electrode is 1500 μm² (2.5×L μm² if the resonatorlength is L (μm)). As a result, a semiconductor laser having a highelectrostatic withstand voltage, resistant to a power supply surge andhaving improved long-term reliability can be obtained.

Tenth Embodiment

FIG. 18 is a bottom view of a semiconductor laser according to TenthEmbodiment of the present invention. This semiconductor laser is formedby cleavage to have a resonator length of 600 μm and separated to have awidth of 400 μm. The element is separated at positions on highdislocation regions 14. Therefore, high dislocation regions exist atopposite sides of the element.

A back electrode 13 for supplying a current has a length of 600 μm inthe resonator direction but has an extremely small width of 10 μm. Thearea of the surface electrode 12 is 100×L μm² if the resonator length isL (μm).

Further, back dummy electrodes 18 electrically insulated from the backelectrode 13 are formed below the substrate at certain distances fromthe back electrode 13. No current is caused to flow through the backdummy electrodes 18. At the time of bonding of the back electrode 13side to a sub mount by soldering, not only the back electrode 13 butalso the back dummy electrodes 18 are bonded to the sub mount.Therefore, the bonding area is increased and the stability with whichthe element is bonded can be improved.

INDUSTRIAL APPLICABILITY

According to the present invention, a semiconductor laser having a highelectrostatic withstand voltage, resistant to a power supply surge andhaving improved long-term reliability can be obtained by reducing acurrent leak through a threading dislocation portion.

1. A semiconductor laser comprising: a substrate having opposite frontand rear surfaces and a high dislocation region having a dislocationdensity of at least 1×10⁵ cm⁻²; a crystalline semiconductor structurelocated on the front surface of the substrate and having an activelayer; an insulating film located on the semiconductor structure; asurface electrode located on the insulating film and electricallycontinuous with the semiconductor structure for injection of a currentinto the active layer; and a back electrode located on the rear surfaceof the substrate, wherein the semiconductor laser includes a laserresonator having a length L, and the surface electrode has an area notexceeding 120×L μm².
 2. The semiconductor laser according to claim 1,wherein the area of the surface electrode does not exceed 80×L μm².
 3. Asemiconductor laser comprising: a substrate having a high dislocationregion having a dislocation density of at least 1×10⁵ cm⁻²; acrystalline semiconductor structure located on the substrate and havingan active layer; an insulating film located on the semiconductorstructure; a surface electrode located on the insulating film andelectrically continuous with the semiconductor structure for injectionof a current into the active layer; a back electrode located on a rearsurface of the substrate, wherein the semiconductor laser includes alaser resonator having a length L, the surface electrode has an area notexceeding 120×L μm²; and a surface dummy electrode located on theinsulating film, and through which no current flows.
 4. Thesemiconductor laser according to claim 3, wherein the area of thesurface electrode does not exceed 80×L μm².
 5. A semiconductor lasercomprising: a GaN substrate having a high dislocation region having adislocation density of at least 1×10⁵ cm⁻²; a crystalline semiconductorstructure located on the front surface of the GaN substrate and havingan active layer; an insulating film located on the semiconductorstructure; a surface electrode located on the insulating film andelectrically continuous with the semiconductor structure for injectionof a current into the active layer; and a back electrode located on arear surface of the GaN substrate, wherein the semiconductor laserincludes a laser resonator having a length L, and the surface electrodehas an area not exceeding 120×L μm².
 6. The semiconductor laseraccording to claim 5, wherein the area of the surface electrode does notexceed 80×L μm².
 7. The semiconductor laser according to claim 5,wherein the GaN substrate has opposite front and rear surfaces and thecrystalline semiconductor structure is disposed on the front surface ofthe GaN substrate.